The present invention is directed, in general, to power conversion and, more specifically, to a circuit for reducing losses associated with a power converter, a method of operating the circuit and a power converter employing the circuit or the method.
The development of high-efficiency power supplies in combination with a requirement of higher power density is a continuing goal in the field of power electronics. A switched-mode power converter is a frequently employed component of a power supply that converts an input voltage waveform into a specified output voltage waveform. There are several types of switched-mode power converters including an asymmetrical half-bridge power converter.
A conventional asymmetrical half-bridge power converter includes two power switches coupled to a controller, at least one isolation transformer, a voltage balancing capacitor, a rectifier and a filter. The asymmetrical half-bridge power converter generally operates as follows. The first and second power switches conduct current in a complimentary manner, with generally unequal duty cycles, to convert an input DC voltage into an AC voltage to be applied across the isolation transformer. Any DC component of the voltage applied to a primary winding of the isolation transformer is blocked by the voltage balancing capacitor coupled in series with the primary winding of the isolation transformer. The rectifier then rectifies a secondary voltage from the isolation transformer and the filter smooths and filters the rectified voltage to develop an output voltage for delivery to a load. The controller monitors the output voltage of the asymmetrical half-bridge power converter and adjusts the duty cycle of the power switches to ultimately control the output voltage. The output voltage may be maintained at a relatively constant level despite relative fluctuations in the input voltage and the load.
The asymmetrical half-bridge power converter is a well known power circuit topology that may be capable of zero voltage switching (ZVS) operation. A high magnetizing current, usually exceeding twice the load current, however, may be required to attain ZVS operation. In U.S. Pat. No. 5,402,329, entitled, xe2x80x9cZero Voltage Switching Pulse Width Modulated Power Converters,xe2x80x9d which is incorporated herein by reference, Wittenbreder suggests placing an inductor in series with the primary winding of the isolation transformer. One of the ZVS transitions is driven by a combination of the magnetic energy stored in the transformer and the magnetizing energy stored in the inductor. The other ZVS transition is driven by energy stored in the inductor. While the series inductance may allow for ZVS operation with lower magnetizing currents, the inductor may cause spurious voltage spikes across the rectifier during reverse recovery.
Further, one or more of the power switches may be subject to current spikes induced therein by the reverse recovery of the rectifier. Saturable reactors may be used in the secondary circuit to reduce current spiking. However, saturable reactors tend to be larger in size than desired and contribute an appreciable cost factor to the power converter.
Accordingly, what is needed in the art is a circuit that reduces spurious spikes in a power converter that overcomes the deficiencies in the prior art.
To address the above-discussed deficiencies of the prior art, the present invention provides, for use with an asymmetrical half-bridge power converter having a primary switching circuit coupled to a primary winding of a transformer and a rectifier coupled to a secondary winding of the transformer, a reverse recovery circuit a method of operation thereof. In one embodiment, the reverse recovery circuit includes an inductor that reduces current spikes in the primary switching circuit caused by a reverse recovery phenomenon associated with the rectifier. The reverse recovery circuit also includes a diode, coupled to the inductor, that clamps a voltage across the rectifier.
The present invention introduces, in one aspect, a circuit capable of reducing current spikes in the primary switching circuit of a power converter. Additionally, voltage spiking across the rectifier associated with a reverse recovery phenomenon is positively affected. Advantageously, recovering transient energy associated with the reverse recovery phenomenon enhances an energy transfer to a load of the power converter and therefore improves the overall efficiency of the power converter.
In one embodiment of the present invention, the reverse recovery circuit includes an auxiliary winding, coupled to the inductor, that transfers a portion of energy in the inductor to the secondary winding. In a related embodiment, the inductor and the auxiliary winding are series-coupled to the primary winding. These arrangements allow an effective recovery of a portion of the energy in the power converter. In another related embodiment, the diode of the reverse recovery circuit is coupled to a node between the auxiliary winding and the primary winding. In yet another related embodiment, the reverse recovery circuit further includes a second diode coupled to the node. Of course, other circuit configurations may be possible and are well within the scope of the present invention.
In one embodiment of the present invention, the power converter further includes a controller that controls conduction intervals of a power switch of the power switching circuit. In a related embodiment, the controller monitors an output voltage of the power converter and to operate the power switch in response thereto. The output voltage of the power converter may thus be regulated despite variations in the input voltage or the load. Of course, the controller may monitor other characteristics associated with the power converter as desired and control the power switch as a function thereof.
The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.